The present invention relates to the production of metal, such as aluminum, and the like, by electrolysis of a compound of the metal which is dissolved in a molten solvent, and more particularly, to a method for reducing in the electrolysis, the corrosion of non-consumable anodes which usually contain oxide compounds and, in which electrolytic process, oxygen gas is caused to evolve.
Presently, the Hall-Heroult Process, as described in "Aluminum Smelter Technology", Aluminum Verlag Duesseldorf, by K. Grjotheim and B. J. Welch, (1980), is commonly used to produce aluminum. Aluminum oxide (alumina) is dissolved in cryolitic melt and electrolyzed in an electrolytic cell. Aluminum forms cathodically at the molten metal pool surface and collects on the carbonaceous cell bottom. Carbon anodes, suspended in the melt, participate in an anodic reaction to form carbon oxides at the anodic surface, resulting in the carbon anode continuously being consumed.
Recently, electrolytic cells were developed in which inert or non-consumable anodes are substituted for the carbon anodes. In such cases, the anodes are not consumed, per se, in the same manner as the carbon, anodes and oxygen gas which can be salvaged for future use is liberated from the decomposition of the alumina. However, the molten bath and the produced oxygen gas cause the anodes to corrode and dissolve in the melt.
The use of inert anodes has several advantages as compared to the consumable carbon anodes and, therefore, is most attractive to the industry. These advantages are set forth in U.S. Pat. No. 3,930,967 issuing to Hanspeter Alder on Jan. 6, 1976, and are briefly stated as: (1) eliminating the need for maintaining a separate manufacturing plant for the production of carbon anodes; (2) eliminating the use of petroleum coke with a low ash content in order to lessen the contaminants in the produced aluminum; (3) creating a stable geometry for the anode thereby permitting a more energy-efficient electrode configuration and cell design, especially if the non-consumable anodes are combined with dimensionally stable cathodes which are presently being developed; and (4) lessening the need for replacing the anodes which allows the electrolytic cell to be operated more consistently as to result in greater aluminum production with fewer undesirable environmental emissions.
The composition, preparation and design features for such non-consumable anodes have been described by several authors, as has been summarized by K. Billehaug and H. A. Oye in "Inert Anodes for Aluminium Electrolysis in Hall-Heroult Cells", Aluminium, Volume 57, pp. 146-150 and pp. 228-231, (1981).
Generally, anodes whose working surfaces are ceramic oxides are being employed. In addition, improvements to the basic ceramic oxide anode are evolving, such as Cermet materials containing oxide and metal. Compositions based on SnO.sub.2 as the main component have been described in early U.S. Pat. Nos. 3,718,550, 3,930,969, 3,960,678, and 3,974,046; Fe.sub.2 O.sub.3 -Ni0 mixtures have been outlined in U.S. Pat. No. 4,374,761; some of these compositions have also been mentioned along with ferritic compositions, i.e. MnFe.sub.2.04 O.sub.4, (MnZn)Fe.sub.2.04 O.sub.4, NiFe.sub.2.04 O.sub.4, by James M. Clark in a recent publication entitled "Anodic Corrosion of Sintered Oxide Materials in Hall-Heroult Melts", at a meeting of the Electrochemical Society held in Washington, D.C. on Oct. 12, 1983. As is apparent, all of these non-consumable electrodes or anodes are essentially composed of sintered ceramic oxides with electronic conductance. Since in the electrolytic process, oxygen gas and molten bath and their motion create a harsh environment, the efficiency and life of these oxidic anodes is eventually impaired in that these anodes are susceptible to corrosion through chemical reactions and dissolution in the melt.
The performance and life of inert anodes such as those outlined in the preceding paragraph is, under practical operating conditions, essentially determined by their dissolution rate occurring at the anode surface, which dissolution rate, in effect, is equated to the corrosion rate of the inert anode. This dissolution rate depends on the mass transfer conditions at the anode, on the saturation concentrations of the anode constituent oxide compounds in the electrolyte melt, and on the actual concentration of these anode constituent compounds in the electrolyte bulk or overall liquid during electrolysis. The mass transfer conditions are affected by the shape of the electrode, and more importantly, by the gas evolution, the amount of oxygen gas evolved being affected by current density and the gas bubble size being affected by electrolyte composition and the dimension of the anodic surface. Electrolytic gas evolution at the electrodes generally produces very high mass transfer rates.
The saturation concentrations of electrode constituent compounds in the melt depend on the type of compounds employed and on the electrolyte composition and temperature, i.e. a high aluminum oxide content for the melt generally decreases the solubility rate of oxidic compounds, which compounds are normally electrode constituent compounds.
The bulk concentrations of anode constituent compounds existing in the electrolyte melt are essentially a result of the dissolution rates of such compounds at the anode or from other sources and of their removal rate from the electrolyte, the latter process mainly occurring by the electrolytic reduction at the cathode where in some instances a partial subsequent reoxidation may occur.